16 



THE CERATOPSIA. 



faces, but which in life were doubtless covered with heavy cartilaginous pads. The basioc- 

 cipital processes are connected distally by a rather thin lamina, and posteriorly there is a deep pit 

 just beneath the occipital condyle. In front they abut against the expanded posterior borders 

 of the processes of the basisphenoid, with which they are united by suture only except in very old 

 individuals, in which they become coossified and the sutures become more or less perfectly 

 obliterated as the age of the animal increases. The basioccipitals are pierced by no foramina. 

 The exoccipitals are large and massive. They entirely inclose the foramen magnum and 

 expand laterally into greatly elongated processes, which project outward and backward, over- 

 lapping the blades of the quadrates and articulating distally with the squamosals by a deep 

 groove on the inferior surface of the latter. The superior border of the distal end of each 

 exoccipital process is expanded and projects into a corresponding notch on the inferior surface 

 of the antero-inferior angle of the parietal. From this notch the suture between the parietals 

 and exoccipitals describes a gentle curve until the supraoccipital is reached, when the superior 



Fig. 6. — Occipital region of Trieeratops flabellatus Marsh (type) , No. 1821, Yale Museum. Inferior view as seen obliquely from behind, a, 

 Postfrontal buttress of supraoccipital; so, supraoccipital; pa, parietal; sq, squamosal; exo, exoccipital; fm, foramen magnum; oc, occipital 

 condyle; bo, basioccipital; bs, basisphenoidal processes; j, jugal; qj, quadratojugal; x, xi, foramen for exit of tenth and eleventh nerves; 

 xil, foramen for exit of twelfth nerve. One-eighth natural size. 



border of the exoccipital is produced into an obtuse angle embraced by the parietal and supra- 

 occipital. The exoccipitals and supraoccipital unite by a nearly horizontal suture. The pos- 

 terior surface of these bones is deeply excavated, inclosing two deep pockets separated by a 

 thin median partition or keel, which becomes stronger above the supraoccipital suture. On 

 the inferior surface and at the base of the lateral or transverse process of the exoccipital there 

 are two foramina, situated one above the other when the skull is in its normal position. The 

 upper and smaller of these two foramina pierces the exoccipital and enters the foramen mag- 

 num just within- the external opening of the latter. It doubtless served to transmit the hypo- 

 glossal, or twelfth nerve. The more inferior but larger foramen divides, one branch, the posterior, 

 opening into the foramen magnum a little in advance of that for the hypoglossal nerve, prob- 

 ably served for the transmission of the eleventh, or accessory, nerve, while the more anterior 

 branch, which opens into the foramen lacerum posterius, may have served for the passage of 

 the pneumogastric nerve. The sutures between the exoccipitals and alisphenoids evidently 



